Microcircuits are well known electrical components that combine hundreds or thousands of individual circuit components and connections in a small volume. The package that holds a typical microcircuit may be no larger than 10 mm. square by 1 mm. thick. One common type of container for a microcircuit called a leadless package, has small connector or contact pads along the periphery of one surface of the package. A single package may have several dozen contact pads by which power is supplied to the microcircuits and signals sent to and from the microcircuit. The contact pads are soldered onto the conductors of a circuit board during assembly of the electrical device.
Before a microcircuit is soldered onto a circuit board, the microcircuit must be tested to assure design functionality. Soldering a defective microcircuit onto a circuit board often ruins the entire board, since typically it is either not possible or not economic to remove a defective microcircuit from a circuit board. Since typical microcircuits are the result of a complex manufacturing process, testing is essential to assure that every microcircuit is completely functional.
For a number of reasons, testing these microcircuits is complex. In the first place, one should not solder the microcircuits to be tested into the test fixture because the act of removing the microcircuits when testing is complete might itself damage the microcircuit.
Secondly, the microcircuits are small and the contacts are closely spaced, on perhaps as small as a 0.3 mm pitch. The contacts themselves may be as small as 0.15 mm wide. For accurate testing, the test fixture contacts must make reliable, low-resistance contact with each of the microcircuit contacts during the entire test process, which may extend to even many hours. Failure to make proper contact with each microcircuit contact for the entire test sequence results in a test that incorrectly fails the microcircuit.
While it is important to test each microcircuit thoroughly, it is also important to test them quickly and cheaply. Accordingly, automated testers have been developed that operate with little human intervention to reliably test hundreds or thousands of individual microcircuits per hour.
A typical tester has its own circuit board with one or more arrays of test contacts that are spaced and aligned to make temporary mechanical contact with the connector pads on the microcircuit package. Each test socket contact is designed to resiliently deflect a very small amount when force is applied. This accommodates any dimensional variations in either the microcircuit package or the test socket contacts.
An alignment plate is mounted on the tester circuit board with an aperture that receives and precisely positions each microcircuit to be tested so that each of the microcircuit contact pads is in precise alignment with the corresponding test contact. The alignment plate is typically bolted to the contactor which is mounted to tester circuit board.
To assure reliable and low resistance electrical conduction between each test socket contact and the corresponding microcircuit contact, the tester includes a presser or loader element that applies sufficient force to the microcircuit package so that each of the microcircuit package contacts at least slightly deflects the corresponding test socket contact. For example, if the test procedure requires 50 grams of force between each package contact and each tester contact, a package with 100 contacts will then require 5 kg. of force for proper electrical connection between each of the microcircuit contacts and the corresponding tester contact. It is not at all convenient to manually apply such a force for the entire duration of a longer test.
Two temporary situations arise where automated force application to the microcircuit during testing is not convenient. One of these situations is where a tester fails more than the expected percentage of microcircuits, or where the tester is inconsistent in testing the same microcircuit. In either case the tester itself must be tested or repaired.
The second situation is when a tester is being first set up for testing a particular microcircuit. During either of these situations, it is necessary for the entire duration of the test to consistently apply the appropriate force to the package to create the required force between the microcircuit contacts and the tester contacts. We find in these circumstances that using the tester loader element is inconvenient and applying force by hand to the package insufficiently precise and very difficult.
Accordingly, a mechanism for temporarily applying a consistent force to press a microcircuit against tester contacts allows an operator to more efficiently deal with each of these two situations.